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After Its Epidemic Arrival, SARS Vanishes

BEIJING, May 14 - Two and a half years after a mysterious respiratory illness from southern China infected thousands of people around the world and brought dire predictions of recurring and deadly plague, the virus known as SARS has again provided a surprise.

It has disappeared, at least for the moment.

Not a single case of severe acute respiratory syndrome has been reported this year or in late 2004. It is the first winter without a case since the initial outbreak in late 2002.

In addition, the epidemic strain of SARS that caused at least 774 deaths worldwide by June of 2003 has not been seen outside a laboratory since then. SARS is not even the nastiest bug in its neighborhood, as health officials warn that avian influenza in Southeast Asia poses a far greater threat.

In cities like Guangzhou and Beijing, once under a state of alert because of SARS, public hysteria about the disease has long since given way to public nonchalance. "Very few people talk about it anymore," said Cheng De, 22, as he walked through a subway tunnel last month in Guangzhou, the city at the center of the first two SARS outbreaks. "People think it is in the past."

Health officials in China are also less alarmed, but they warn that SARS could still pose a threat. This caution partly reflects the lack of knowledge about the virus: What caused it to become so virulent in the initial outbreak? Where has it gone? Will it come back?

Most researchers and health officials are not counting on the rosiest scenario -- that SARS has simply mutated into oblivion.

"We'd be lucky to believe that, and that would be very nice, but there is no research to support that," said Dr. Julie Hall, the SARS team leader at the Beijing office of the World Health Organization. "Just because we've not seen SARS anymore this year doesn't mean it is not out in the wild this year."

Health officials have categorized SARS into three known outbreaks: the worldwide epidemic of more than 8,000 cases that began in November 2002 and ended in June 2003; the second outbreak from December 2003 through January 2004 that involved a milder strain of the virus and caused only four cases; and the nine cases traced to laboratory accidents in China, Taiwan and Singapore between March and May of last year.

Scientists agree that SARS jumped from animals to humans, probably in wildlife markets in the region around Guangzhou, where workers live near the animals they slaughter and sell. In January 2004, Chinese officials ordered a nationwide culling of civet cats from restaurants and wildlife markets after Chinese scientists concluded that the animal was the primary source of the outbreaks. The small, weasel-like animal is considered a delicacy in southern China.

"This year nothing happened because we have very, very strong rules now controlling the wildlife markets," said Dr. Zhong Nanshan, China's leading SARS expert, who advocated the cull of civets.

Dr. Zhong, director of the Guangzhou Institute of Respiratory Diseases, said new tests of wild civets from northern China found that none had been exposed to SARS, but as the animals moved closer to the wildlife markets in Guangzhou, the ratio of those exposed to the illness climbed rapidly.

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"There must be something happening in the transportation of civets from the north to the markets in Guangzhou," Dr. Zhong said.

He said his researchers were still trying to determine why civets were so susceptible to the disease and how, specifically, it jumped to humans. It is also unclear what other animals, if any, are carriers of the disease. Rats were initially suspected in the 2003 outbreak in Hong Kong. Ultimately, SARS had a fatality rate of 10 percent, though for people 60 and older the death rate rose to 50 percent.

This year, Dr. Kathryn V. Holmes, a prominent microbiologist who has studied coronaviruses like SARS for more than 20 years, caused an immediate stir in China after giving a speech at the annual meeting of the American Association for the Advancement of Science. News media reports in China quoted Dr. Holmes as saying that SARS no longer existed in the wild and that the virus no longer presented a serious health threat to the world.

In a telephone interview recently, Dr. Holmes, a professor at the University of Colorado Health Sciences Center in Denver, said her comments should never have been interpreted to mean that SARS no longer existed in nature. Instead, she said she was referring to recent research showing that the epidemic strain of SARS that passed from person to person during the initial worldwide outbreak had not been seen since June 2003. This strain mutated after passing from animals to humans in a way that caused it to spread rapidly and become more virulent.

"People are trying to understand which of those mutations was responsible for human-to-human transmission and high virulence in humans," Dr. Holmes said of one of the fundamental unanswered questions about SARS.

Dr. Holmes credited the fact that SARS had "vanished" this year to the aggressive containment efforts by the World Health Organization and the Chinese government.

"The vanishing was a piece of outstanding coordination throughout the world," she said. "They controlled the epidemic without having a good diagnostic test for the virus."

Research on different SARS vaccines is under way at several laboratories in China and elsewhere. Dr. Hall, the World Health Organization expert in Beijing, said the most reassuring discovery about SARS was that it could be contained. But, she noted, the fact that the disease spread quickly throughout the world also showed the potential for more serious diseases like avian flu to cause an epidemic.

"One thing that has come out of SARS is the need to strengthen public health surveillance," Dr. Hall said. "If SARS revealed the gaps in the system, then avian influenza has increased the urgency to fill those gaps."

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A version of this article appears in print on May 15, 2005, on Page 1001008 of the National edition with the headline: After Its Epidemic Arrival, SARS Vanishes. Order Reprints|Today's Paper|Subscribe